This invention relates to the production of a plastics foam which is formed utilising a non-fluorocarbon blowing agent. The description of the invention hereinafter generally refers to the use of carbon dioxide alone as the blowing agent. This is only because this is the preferred agent. Other natural gases can be used with CO.sub.2 such as nitrogen, air and water.
Until recently the favoured agents for use in the generation of plastics extruded foams, have been fluorocarbons such as dichlorofluoromethane and trichlorofluoromethane. In recent times, however, there has been substantial movement away from the use of such compounds as some scientific studies have indicated that fluorocarbons when released into the atmosphere may have damaging consequences, in particular to the ozone layer. Alternative technologies have been developed utilising natural gases as blowing agents such as carbon dioxide and an example can be found in U.S. Pat. No. 4,436,679.
However, there have been very few, if any commercial operations reported using CO.sub.2, as the sole blowing agent in the manufacture of polystyrene foam using molten polystyrene. Most commercial operations using CO.sub.2 as a blowing agent use CO.sub.2 in combination with a hydrocarbon blowing agent such as pentane or butane or with a fluorocarbon product such as Du Pont 152A. It is an object of the present invention to produce a foam using a CO.sub.2 or CO.sub.2 containing blowing agent. Preferably the foam produced should have characteristics and properties at least as good as a fluorocarbon blown foam. Most preferably the foam produced will have an average cell diameter below 22 microns, a cell wall thickness below 4 microns, a density between 1.5 to 3.0 lbs/ft.sup.3 and cells which are substantially uniformly oriented in all three dimensions.
Hitherto, it has been considered that plastic foams such as polystyrene foams when blown using a natural gas should incorporate a fairly low molar amount of the blowing agent entrained in the resin. It has been previously considered that generally as the proportion of blowing agent is increased the density of the foam is decreased and that the physical strength of the foam decreases with the density. For example polystyrene foams produced using above about 0.1 moles blowing agent per 10 g of polystyrene have generally been considered to be too weak to be of commercial value, particularly when the foam is intended for use in making an end product such as a tray or other supporting substrate. It has also been previously considered that the control of the foaming process is more difficult as the proportion of blowing agent is increased. Most examples previously published suggest the use of a proportion of CO.sub.2 (when making a polystyrene foam) in the range 1.5-3.0% by weight (0.034-0.068 moles CO.sub.2 /100 g polystyrene).
These previously made foams have been extruded at a stock at die temperature of about 140 to 155.degree. C. It has also been found with these foams that if they are extruded at the bottom end of this temperature range they have a tendency to shrink immediately after manufacture. This is probably due to the rapid diffusion of CO.sub.2, out of the cells causing a partial vacuum in the cells. Shrinkage may be controlled by increasing the stock at die temperature or by decreasing the CO.sub.2 concentration.